In a conventional laser beam printer, writing has been carried out by using an optical scanning apparatus having a scanning optical system that scans a scanning surface by deflecting a light flux emitted from a semiconductor laser with a polygon mirror that rotates at constant speed, and by converging the light flux on the scanning surface.
As the scanning optical system of this kind, there is known a scanning optical system employing a light source whose wavelength λ is 780 nm and a polygon mirror having six surfaces, as disclosed, for example, in Unexamined Japanese Patent Application Publication (JP-A) No. 2006-337916.
In recent years, a laser beam printer providing higher speed of printing and higher definition of printing is demanded.
For attaining this higher speed, it is considered increasing the number of reflection surfaces of the polygon mirror and increasing writing frequencies per one revolution of the polygon mirror. For attaining this higher definition of printing, it is considered decreasing a beam diameter on the scanning surface which corresponds to an image surface.
On the other hand, there is a following relationship in the scanning optical system, where Φ represents a beam diameter in the main scanning direction on the scanning surface corresponding to an image surface, λ represents a wavelength of a light source, f represents a focal length in the main scanning direction of an image-forming lens group arranged between the polygon mirror and the scanning surface, and G represents a diameter in the main scanning direction of a light flux entering the image-forming lens group.Φ=C×λ×f/G  (6)
In the expression, C is a constant of proportionality.
Further, there is a following relationship in the scanning optical system, where θ represents an angle formed by a light flux reflected by the polygon mirror and an optical axis of the image-forming lens group, and Y represents a distance from the point of intersection of the scanning surface with the optical axis of the image-forming lens group to a beam on the scanning surface.Y=f×θ  (7)
If the number of reflection surfaces of the polygon mirror is increased simply, the maximum angle of scanning per one reflection surface becomes smaller. Namely, the maximum value of the angle θ becomes smaller, and it is necessary to set focal length f of image-forming lens group to be larger from expression (7), for securing the same scanning width. Therefore, the scanning optical system is made to be greater, which is a problem.
If focal length f grows further greater, there occurs a problem that beam diameter Φ becomes greater due to the expression (6), and higher definition of printing becomes more difficult. If diameter G of a light flux in the main scanning direction entering the image-forming lens group is established to be great, for making beam diameter Φ to be small, there are caused problems of a side increase of the polygon mirror, a cost increase of a polygon mirror, and of a cost increase caused by a use of a motor that withstands a load including a mass increase of the polygon mirror.
In the scanning optical system of JP-A No. 2006-337916, a light flux with wavelength of 780 nm is used, the number of reflection surfaces of a polygon mirror is 6 and a focal length of an image-forming lens is 150 mm. When a scanning width in the direction of a short side of A4-sized sheet is 236 mm in the scanning optical system, an angle for one reflection surface of the polygon mirror to incline a light flux is 45.1°, and the number of reflection surfaces that can be formed on the polygon mirror is 7 at the maximum. Further, if the number of reflection surfaces is increased by causing a focal length of the image-forming lens group to be greater than 150 mm, there is a problem that a circumscribed-circle diameter of the polygon mirror grows greater.
Namely, it has been difficult to attain low cost, high speed printing and high definition of printing, when the polygon mirror having the increased number of reflection surfaces is used.